1. Field of the Invention
The present invention relates generally to a method for forming a thermoplastic article having a highly developed strain crystallized morphology. More particularly, one phase of the method involves stretching the thermoplastic article at a temperature below its glass transition temperature prior to a final forming operation, such as blow molding.
2. The Prior Art
The prior methods of forming thermoplastic articles in blow molding operations have typically involved either (1) heating a parison to a molecularly orientable temperature and then blowing the parison, (2) heating a parison to a molecularly orientable temperature, axially stretching the parison, and then blowing the stretched parison, or (3) blowing a parison to the configuration of a pre-form and then blowing the pre-form to the final article. These methods, as applied to crystallizable thermoplastic materials, have yielded blown containers where the material has been crystallized on the order of only about 10 to 12% generally. Since the degree of crystallinity affects several property characteristics, such as permeability, creep resistance, tensile strength, etc., it is desirable to enhance the degree of crystallinity without significantly extending cycle times in a blow molding operation.
Thus, one primary object of the present invention is to form a blown product which has improved property characteristics over those products formed by the currently recognized forming techniques.